svm: support VMCB cleanbits

svm: support VMCB cleanbits

Attached patch implements the VMCB cleanbits SVM feature.
Upcoming AMD CPUs introduce them and they are basically hints
for the CPU which vmcb values can be re-used from the previous
VMRUN instruction.

Each bit represents a certain set of fields in the VMCB.
Setting a bit tells the cpu it can re-use the cached value
from the previous VMRUN.
Clearing a bit tells the cpu to reload the values from the given VMCB.

Signed-off-by: Wei Huang <Wei.Huang2@amd.com>
Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>
Signed-off-by: Keir Fraser <keir@xen.org>